Method and apparatus for hot pressing

ABSTRACT

A method and apparatus for generating pressures wherein a highly anisotropic pyrolytic graphite body having its highest coefficient of linear thermal expansion in the direction of an axis thereof is confined in a pressing vessel together with a compactible material which is positioned along said axis. Heating of the anisotropic body expands the same along said axis to compress the compactible material. The pressing vessel is composed of a material having a low coefficient of thermal expansion relative to the anisotropic body. Preferably, the pressing vessel is composed of isotropic graphite, so that high pressures can be generated by furnacing the entire assembly at temperatures as high as 2200* C.

United States Patent [72] Inventor .11 Young Chang [56] References Cited [21] A l N gaigyigge, Tenn. UNITED STATES PATENTS o. [22] 521 Sept 25, 1969 2,585,818 2/1952 Moravec 219/50 [45] Patented Mar. 2, 1971 Primary Examiner-J. V. Truhe [73] Assignee the United States of America, as Assistant Examiner-Hugh D. Jaeger t d by th U it d st t At i Attorney- Roland A. Anderson Energy Commission ABSTRACT: A method and apparatus for generating pressures wherein a highly anisotropic pyrolytic graphite body having its highest coefficient of linear thermal expansion in the direction of an axis thereof is confined in a pressing vessel together with a compactible material which is positioned [54] METHOD AND FOR HOT PRESSING along said axis. Heating of the anisotropic body expands the scmm" 1 3 same along said axis to compress the compactible material. [52] U.S.Cl 219/50, The pressing vessel is composed of a material having a low 18/16.5,2l9/6.5 coefficient of thermal expansion relative to the anisotropic [51] Int. Cl. 1105b 1/00 body. Preferably, the pressing vessel is composed of isotropic [50] Field of Search 219/50, 65; graphite, so that high pressures can be generated by fumacing 18/165 (High Pressure Digest) the entire assembly at temperatures as high as 2200C.

METHOD AND APPARATUS FOR HOT PRESSING' BACKGROUND OF THE INVENTION This invention relates to a method and apparatus for hotpressing compactible materials. The invention was made in the course of, or under, a contract with the US. Atomic Energy Commission.

Powders and other compactible materials commonly are densified by the simultaneous application of heat and pressure to achieve product densities at pressures must lower than would be required at room temperature. Conventional hotpressing is not, however, well adapted to the mass production of densified products because conventional hot-presses are comparatively complex, expensive and slow-operating.

US. Pat. No. 2,585,818 (to Moravec) describes a pressing device which is comparatively simple, inexpensive, and adapted to generate a predetermined pressure when heated to a selected temperature within a limited range. That kind of press facilitates hot-pressing on a mass-production basis in the sense that a plurality of such presses can be loaded with similar samples and then heated simultaneously in a common furnace to produce essentially identical pressings.

The pressing device described in the above-mentioned patent includes a pressing chamber in which a substantially isotropic metal bar (e.g., a steel bar) and the material to be densified are confined in abutting relation. The bar is selected to have a coefficient of thermal expansion substantially higher than that of the remainder of the press and that of the material to be densified. Heating of the loaded die causes relatively great expansion of the metal bar, which consequently exerts pressure on the sample.

While the above-mentioned device permits the generation of high pressures, it is not well adapted for use at very high temperatures in the sense that few metals are available which are characterized by the combination of a relatively high melting point and a relatively high coefficient of linear thermal expansion (CTE). For example, ferro-nickel has a CTE of 18 X 10- (increase in length per unit length, measured at 0 C., per degree Centigrade), but it has a melting point of 1500 C. Stellite Alloy No. 6 has a CTE of 17 X but a melting point of l,275 C. Steel containing 1.2 percent carbon has a CTE of about 29 X 10- but a melting point of -l,335 C. Thus, devices of the kind described in the above-"mentioned patent are not suitable for use at very high temperatures.

It is, therefore, an object of this invention to provide a comparatively simple and inexpensive device for generating pressures useful in the compaction of materials. It is another object to provide a device for generating pressures which are a function of temperature. It is still another object to provide a device for generating pressures at temperatures as high as 2,200 C.

SUMMARY OF THE INVENTION This invention relates to a method and an apparatus for the hot-pressing of compactible materials. The method comprises confining a compactible material and an anisotropic pyrolytic graphite body within a chamber in a pressing vessel. The anisotropic body has its highest coefficient of linear thermal expansion along an axis thereof, and the compactible material is positioned to intercept this axis. The pressing vessel is selected to have a coefficient of thermal expansion which is low relative to that of the anisotropic body along said axis. Heat is applied to said anisotropic body to expand the same along said axis and compress the compactible material.

In a preferred form of the invention the pressing vessel is formed of substantially isotropic graphite, and pressing is accomplished by furnacing the entire'assembly at temperatures up to about 2,200C.

BRIEF DESCRIPTION OF THE DRAWING The FIGURE is a cutaway pictorial view of a pressing assembly designed in accordance with this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the FIGURE, a preferred embodiment of this invention utilizes a cylindrical pressing q vessel 1 which is composed of a substantially isotropic graphite, such as ATJ graphite (produced by the NationalCarbon Company) or H337 graphite (produced by the Great Lakes Carbon Corporation). The ATJ graphite is a fine-grain high-density (1.73 g./cm. graphite having a linear coefficient of thermal expansion of 2.19 to 3.42 X 10- in./in./ C. As shown, the vessel 1 is formed with an axially extending cylindrical pressing chamber 2. Blocks 3, 4, and 5 of a highly anisotropic pyrolytic graphite are stacked one on another on the bottom of the chamber, with their c-axes (the axes characterized by the highest coefficient of expansion) parallel with the major axis of the chamber. Mounted on the topmost block 5 is a cylindrical anvil 6 which extends upward into a close-fitting sleeve 7 to contact a compactible material 8 loaded therein. A similar anvil 9 extends downwardly into the sleeve 7 to contact the compactible material. Preferably, the components 6, 7, and 9 are composed of isotropic graphite. Pyrolytic graphite blocks 3, 4, and 5, axially oriented likethose previously described, are stacked on the upper anvil 9. As shown, the upper end of the vessel 1 is cut away to form a comparatively large opening defined by an inwardly sloping wall. 10 adjoining chamber 2 and a threaded cylindrical wall 11 adjoining the top face of the vessel. An externally threaded plug 12 is screwed into the threaded wall 11. The base of the plug 12 is contoured to extend into the upper end of chamber 2 and bear on the graphite body 3' so as to prevent outward movement of the components in the chamber 2. The vessel 1 or the plug 12 is traversed by one or more small passages 13 communicating with the chamber 2; these permit the gas pressure in the chamber 2 to come to equilibrium with the furnace atmosphere during furnacing.

The graphite blocks referred to above may be composed of various commercially available pyrolytic graphites having a comparatively high coefficient of thermal expansion along the c-axis. One such graphite, manufactured by the General Electric Company, is characterized by a c-axis CTE of 22 X 10- in./in./ C. and a compressive strength of 54,000 to 82,000 p.s.i. at 25 C. Another highly suitable pyrolytic graphite, manufactured by the Space-Age Materials Corporation, has a c-axis CTE of 13 X 10- in./in./ C. (25? C. and 2760 C.) and a compressive strength of 60,000 p.s.i. at 25 C. Various other highly anisotropic graphites suitable for this application are commercially availablefor example, Type TSX (manufactured by the National Carbon Company) and an undesignated pyrolytic graphite manufactured by High-Temperature Materials, Inc.

In a typical operation of the pressing assembly illustrated in the FIGURE, the pyrolytic graphite blocks 3, 4, and 5 are stacked on the bottom of the chamber 2, with their c-axes parallel with or coincident with the major axis of the chamber. Before insertion in the vessel- 1, the lower anvil 6 is fitted in the lower end of the sleeve 7, and the latter is loaded with a selected amount of the material 8 to be compacted. This material may be loose powder, but preferably is a partially compacted body, such asa cold-pressed compact. The upper anvil 9 then is inserted in the upper portion of the sleeve against the compact. This assembly (components 6, 7, 8, 9) then is positioned on the graphite block 5, after which the blocks 3, 4, and 5' are stacked in the chamber as described. The plug 12 then is screwed in place to bear against block 3':

Hot-pressing of the material 8 is accomplished by gradually heating the pyrolytic graphite blocks to a selected temperature. This may be accomplished in various ways, a very convenient technique being to heat the entire vessel 1. in a controlled-temperature furnace 14. If desired, the vessel might be heated by induction, in which case heating is accomplished with an AC-powered induction coil 15 encircling the vessel. The arrangement also can be modified to permit heating by conduction. For example, the lower end of the vessel 1 can be cut away to receive a plug similar to plug 12, and the vessel 1 can be insulated electrically from the remainder of the components. Assuming that the other components have been fabricated from electrically conductive materials, a heating current can be passed from one'plug to the other, through the components in chamber 2. The material 8 may not be electrically conductive, but the sleeve 7 -will complete the circuit between the upper and lower halves of the assembly.

EXAMPLE 1 Uranium nitride powder was cold-pressed at 50 tons per square inch (50 TSI) to forma right-cylindrical pellet having a a density of 8.41 gJcmF, a diameter of 0.370 inch, and a length of 0.035 inch. The pellet was hot-pressed in an assembly of the kind illustrated in the FIGURE. The vessel 1 was composed of Type ATJ isotropic graphite, identified above. The graphite blocks 3, 4, and 3', 4', 5' were composed of the General Electric Company graphite previously described. The anvils 6, 9 and thesleeve 7 were composed of Type AXF-EP-1924 isotropic graphite, manufactured by POCO Graphite, Inc. The plug 12 was composed of Type ATJ isotropic graphite.

The pressing assembly was placed in an argon-atmosphere graphite furnace and heated to 1,500 C. for a period of 40 minutes. The pellet subsequently was examined and was found to have a density exceeding 99 percent of the theoretical. When uranium nitride powder is cold-pressed at about 30 TSI and then sintered at about l,590 C., a density of only about 73 percent theoretical is obtained.

EXAMPLE [I Aluminum oxide powder was cold-pressed at 20 TSI to form a pellet measuring 0.371 inch by 0.052 inch. The pellet was mounted in an assembly of the kind employed in Example I. The assembly was furnaced at l,600 C. for 40 minutes. The resulting sintered compact was characterized by a comparatively small grain size and had a density of 99+ percent of the theoretical.

EXAMPLE III I Ten grams of alumina was blended with 0.047 gram of chromic acid to form a mixture which was cold-pressed at 30 TSI to form a pellet. The pellet was loaded in an assembly of the kind employed in Example I and was furnaced at l,6000 synthetic C. for 40 minutes. The resulting sycthetic sapphire had a density of 99+ percent of theoretical and a hardness of 9 on Mohs scale. The synthetic sapphire was heat-treated for 22 hours at l,750 C. and etched with hydrogen. Photomicrographs of the etched materialshowed porosity.

Pressing in accordance with this invention can be conducted at temperatures as high as 2,200 C. when the entire pressing assembly is composed of graphite. Good results are obtained, however, if the pressing components other than the pyrolytic graphite blocks are composed of various materials other than graphite, so long as they have suitable strength at elevated temperatures and so long as their linear coefficient of thermal expansion in the direction of the major axis of the chamber 2 is low compared to that of the pyrolytic blocks along their c-axis. Tungsten is especially suitable in this regard, but other materials such as refractory metal oxides, nitrides, and borides are satisfactory.

The above-mentioned pyrolytic blocks must be oriented axially as described. The use of a plurality of the blocks is not essential if a single block of suitable dimensions is available. With the exception of the sleeve 7, the components confined in the chamber 2 are disposed in serially abutting relation. Thus, growth of the pyrolytic graphite along the c-axis is effective in compressing the sample whether or not the pyrolytic graphite directly abuts the sample. In applications where the pyrolytic graphite might react with the sample being compressed, a nonreactive refractory body (e.g., tungsten, in the case of uranium nitride) may be interposed between the pyrolytic graphite and the sample.

Compare to the above-c1ted patented device, the use of an anisotropic pressure-generating material permits an appreciable reduction in the radial strength requirement for the wall defining the pressing chamber.

As mentioned, hot-pressing in accordance with this method is especially suitable for the production of many articles of like density, since a single furnace can be used to heat a plurality of the comparatively simple pressing assemblies at one time.

The foregoing description is intended to be illustrative of my invention, the scope of which is to be limited only by the appended claims.

Iclaim:

1. The method of hot-pressing a compactible material which comprises tightly confining within a chamber of a pressing vessel said material and an anisotropic pyrolytic graphite body having in the direction of an axis thereof a coefficient of thermal expansion which is greater than the corresponding coefficients of thermal expansion for said material and the walls defining said chamber, said material being positioned to intercept said axis of said anisotropic body, and heating said anisotropic body to expand the same along said axis to compress said material.

2. The method of claim 1 wherein said heating is effected by furnacing said pressing vessel containing said material and said anisotropic body.

3. The method of claim 1 wherein saidheating is effected by induction.

4. A device for hot-pressing a compactible material comprising:

a. a pressing vessel having a chamber including a volume for receiving a charge of said compactible material;

b. closure means for tightly closing said chamber;

c. an anisotropic pyrolytic graphite body having in the direction of an axis thereof a coefficient of thermal expansion which is greater than the corresponding coefficients of thermal expansion of said material, the walls of said chamber, and said closure means, said anisotropic body being disposed within said chamber with said axis intersecting said volume; and

d. means for heating said anisotropic body to expand the same along said axis into said volume.

5. The device of claim 4 wherein said pressing vessel and said closure means are composed of substantially isotropic graphite.

6. The device of claim 4 wherein said pressing vessel and said closure means are composed of refractory material. 

2. The method of claim 1 wherein said heating is effected by furnacing said pressing vessel containing said material and said anisotropic body.
 3. The method of claim 1 wherein said heating is effected by induction.
 4. A device for hot-pressing a compactible material comprising: a. a pressing vessel having a chamber including a volume for receiving a charge of said compactible material; b. closure means for tightly closing said chamber; c. an anisotropic pyrolytic graphite body having in the direction of an axis thereof a coefficient of thermal expansion which is greater than the corresponding coefficients of thermal expansion of said material, the walls of said chamber, and said closure means, said anisotropic body being disposed within said chamber with said axis intersecting said volume; and d. means for heating said anisotropic bOdy to expand the same along said axis into said volume.
 5. The device of claim 4 wherein said pressing vessel and said closure means are composed of substantially isotropic graphite.
 6. The device of claim 4 wherein said pressing vessel and said closure means are composed of refractory material. 